use github actions cache for docker build

[itcarroll]

Is your feature request related to a problem? Please describe.
The repo2docker-action takes a while, and most of that time is spent building docker layers that have not changed. On a local system, Docker would use its layer cache and take less time.

Describe the solution you'd like
GitHub Actions provides a way to persist a cache from a runner, and Docker appears to have documentation about using it at: https://docs.docker.com/build/cache/backends/gha/. Could the repo2docker-action integrate such a mechanism, so that Docker images could be built using a cache?